The present invention relates to optical lithography, and more particularly, relates to a photo-lithography technique that utilizes Talbot sub-images of an illuminated grating to print a line-space pattern in photosensitive media such that the printed pattern has a pitch that is smaller than the mask pitch.
Lithography refers to a family of techniques for transferring an image rendered on one form of media onto that of another media, typically by photographically xe2x80x9cprintingxe2x80x9d the image. Photolithography techniques are widely used in semiconductor manufacture and other applications such as large or ultra-large scale integration semiconductor microcircuits (chips). Silicon substrates upon which these circuits are to be created are coated with a radiation (i.e., light) sensitive material chosen for its ability to accurately replicate the desired image, exposed to a source of radiation partially blocked by a mask having the required pitch to render a pattern. Typically, the circuit pattern is rendered as a positive or negative mask image which is then xe2x80x9cprojectedxe2x80x9d onto the coated substrate, in either a transmission or reflection mode, depending on the type of optical lithography system being used. The mask is thus imaged on the surface of the coated substrate where the incoming radiation chemically changes those areas of the coating (e.g., photosensitive layer) on which the process radiation impinges, usually by polymerizing the coating exposed to the radiation. Depending on the developer (solvent) used the unpolymerized areas are removed, being more soluble in the developer than the polymerized regions, and the desired pattern image remains.
Since this process allows chip manufacturers to effectively replicate the mask image indefinitely with little additional expense, xe2x80x9cprojectionxe2x80x9d photolithography has become an essential and powerful tool for manufacturing semiconductor xe2x80x9cchips.xe2x80x9d However, as the drive to place ever greater numbers for components on those chips continues, the need to resolve ever smaller image features also continues. In doing so, the diffraction limits of visible light wavelengths have been reached. In order to continue xe2x80x9cprintingxe2x80x9d these features with high resolution and contrast, using shorter wavelength radiation is necessary. Typical optical lithography systems use radiation at wavelengths such as 365 nm, 248 nm, 193 nm, 157 nm and 126 nm. Currently, only 193 nm steppers are commercially available for volume manufacturing, however. Steppers using 157 nm and 126 nm wavelengths are still in the development phase. Advanced non-optical lithography systems with shorter wavelengths such as xe2x80x9cextremexe2x80x9d ultraviolet or soft x-ray are now being actively researched for printing complex patterns in the extreme submicron range. However, the problem of diffraction limited optics remains, and the drive to using shorter wavelengths provide only limited results.
In addition to shorter wavelength radiation, there are several techniques available for high resolution and contrast optical lithography. One technique developed by chip manufacturers to increase the resolution and contrast of optical lithography involves the use of phase-shifting masks. Light rays transmitted through adjacent apertures of the mask follow different phases. However, phase-shifting masks are costly and difficult to manufacture because the phase structure must be closely related to specific geometries of the mask pattern. Moreover, as microcircuit pitches shrinks in size, mask making techniques do not necessarily keep pace.
Another technique developed by chip manufacturers is referred as xe2x80x9cengineered illuminationxe2x80x9d to help print smaller and smaller features of semiconductor microcircuits. This technique relies upon the use of various xe2x80x9cpatternsxe2x80x9d of illumination including annular and quadrapole illumination, and off-axis illumination. However, these methods result in reduced condenser efficiency or require that the illuminator be seriously modified. All of these methods and assist features are time consuming, expensive, and less efficient. Advanced non-optical lithography systems such as xe2x80x9cextremexe2x80x9d ultraviolet (EUV) lithography and e-beam (SCALPEL) lithography are being developed; however, they are cost-prohibitive.
Accordingly, there is a need for a simple yet effective method for existing optical lithography systems to print patterns in the submicron range such as nested (or periodic) line-space patterns in the metal and shallow trench regions (STR) with better feasibility and manufacturing tolerance.
Accordingly, various embodiments of the present invention are directed to an optical lithography system and method which utilizes a mask pattern and forms Talbot sub-images of an illuminated grating onto a photosensitive target to print an image pattern on the photosensitive target such that the image pattern printed has a pitch equal to half the pitch of the mask pattern.